ATLANTA — The swine flu outbreak that has alarmed the world for a week now appears less ominous, with the virus showing little staying power in the hardest-hit cities and scientists suggesting it lacks the genetic fortitude of past killer bugs.
A flu expert said he sees no reason to believe the virus is particularly lethal. And a U.S. federal scientist said the germ's genetic makeup lacks some traits seen in the deadly 1918 flu pandemic strain and the more recent killer bird flu.
Still, it was too soon to be certain what the swine flu virus will do. Experts say the only wise course is to prepare for the worst. But in a world that's been rattled by the specter of a global pandemic, glimmers of hope were more than welcome Friday.
In New York City, which has the most confirmed swine flu cases in the U.S. with 49, swine flu has not spread far beyond cases linked to one Catholic school. In Mexico, the epicenter of the outbreak, very few relatives of flu victims seem to have caught it.
Officials in Mexico have voiced optimism for two days that the worst may be over. But Dr. Scott F. Dowell of the U.S. Centers for Disease Control and Prevention said it's hard to know whether the outbreak is easing up in Mexico. "They're still seeing plenty of cases," Dowell said.
He said outbreaks in any given area might be relatively brief, so that they may seem to be ending in some areas that had a lot of illness a few weeks ago. But cases are occurring elsewhere, and national numbers in Mexico are not abating, he said.
A top Mexican medical officer questioned the World Health Organization's handling of the early signs of the swine flu scare, suggesting Thursday that a regional arm of the WHO had taken too long to notify WHO headquarters of about a unusually late rash of flu cases in Mexico.
The regional agency, however, provided a timeline to the AP suggesting it was Mexico that failed to respond to its request to alert other nations to the first hints of the outbreak.
The Mexican official, chief epidemiologist Dr. Miguel Angel Lezana, backtracked Friday, telling Radio Formula: "There was no delay by the Mexican authorities, nor was there any by the World Health Organization."
In Mexico, where swine flu has killed at least 15 people and the confirmed case count has surpassed 300, the health secretary said few of the relatives of 86 suspected swine flu patients had caught the virus. Only four of the 219 relatives surveyed turned up as probable cases.
As recently as Wednesday, Mexican authorities said there were 168 suspected swine flu deaths in the country and almost 2,500 suspected cases. The officials have stopped updating that number and say those totals may have even been inflated.
Mexico shut down all but essential government services and private businesses Friday, the start of a five-day shutdown that includes a holiday weekend. Authorities there will use the break to determine whether emergency measures can be eased.
In the Mexican capital, there were no reports of deaths overnight — the first time that has happened since the emergency was declared a week ago, said Mayor Marcelo Ebrard.
"This isn't to say we are lowering our guard or we think we no longer have problems," Ebrard said. "But we're moving in the right direction."
The U.S. case count rose to 155 on Friday, based on federal and state counts, although state laboratory operators believe the number is higher because they are not testing all suspected cases.
President Barack Obama voiced hope Friday that the virus may turn out to be no more harmful than the average seasonal flu.
"It may turn out that H1N1 runs its course like ordinary flus, in which case we will have prepared and we won't need all these preparations," Obama said, using the flu's scientific name.
The president stressed the U.S. government was still taking the virus very seriously, adding that even if this round turns out to be mild, the bug could return in a deadlier form during the next flu season.
New York officials said after a week of monitoring the disease that the city's outbreak gives little sign of spreading beyond a few pockets or getting more dangerous. Mayor Michael Bloomberg said the outbreak in New York had so far proved to be "a relatively minor annoyance."
Worldwide, the total confirmed cases neared 600, although that number is also believed to be much larger. Besides the U.S. and Mexico, the virus has been detected in Canada, New Zealand, China, Israel and eight European nations.
There were still plenty of signs Friday of worldwide concern.
China decided to suspend flights from Mexico to Shanghai because of a case of swine flu confirmed in a flight from Mexico, China's state-run Xinhua News Agency reported.
And in Hong Kong, hundreds of hotel guests and workers were quarantined after a tourist from Mexico tested positive for swine flu, Asia's first confirmed case.
Evoking the 2003 SARS outbreak, workers in protective suits and masks wiped down tables, floors and windows. Guests at the hotel waved to photographers from their windows.
Scientists looking closely at the H1N1 virus itself have found some encouraging news, said Nancy Cox, flu chief at the federal Centers for Disease Control and Prevention. Its genetic makeup doesn't show specific traits that showed up in the 1918 pandemic virus, which killed about 40 million to 50 million people worldwide.
"However, we know that there is a great deal that we do not understand about the virulence of the 1918 virus or other influenza viruses" that caused serious illnesses, Cox said. "So we are continuing to learn."
She told The Associated Press that the swine flu virus also lacked genetic traits associated with the virulence of the bird flu virus, which grabbed headlines a few years ago and has killed 250 people, mostly in Asia.
Researchers will get a better idea of how dangerous this virus is over the next week to 10 days, said Peter Palese, a leading flu researcher with Mount Sinai Medical School in New York.
So far in the United States, he said, the virus appears to look and behave like the garden-variety flus that strike every winter. "There is no real reason to believe this is a more serious strain," he said.
Palese said many adults probably have immune systems primed to handle the virus because it is so similar to another common flu strain.
As for why the illness has predominantly affected children and teenagers in New York, Palese said older people probably have more antibodies from exposure to similar types of flu that help them fight off infection.
"The virus is so close," he said.
In the United States, most of the people with swine flu have been treated at home. Only nine people are known to have ended up in the hospital, though officials suspect there are more.
Obama said the U.S. government is working to produce a vaccine down the road, developing clear guidelines for school closings and trying to ensure businesses cooperate with workers who run out of sick leave.
He pointed out that regular seasonal flus kill about 36,000 people in the United States in an average year and send 200,000 to the hospital.